Thermocouple extension wire

ABSTRACT

A thermocouple extension wire composition comprising, as expressed in percent by weight; nickel 16- 20 percent; manganese 1.30- 2.00 percent; cobalt 0.70- 1.70 percent; balance copper. A pair of thermocouple extension wires, one wire having the above composition, the other wire being composed essentially of iron, exhibit a differential electromotive force substantially equal to the electromotive force developed by Type K thermocouple between 32* to 400* F.

United States Patent Teh Po Wang Cedar Grove, NJ.

Dec. 16, 1969 Dec. 21, 1971 Wilbur B. Driver Company Inventor Appl. No. Filed Patented Assignee THERMOCOUPLE EXTENSION WIRE 1 Claim, 1 Drawing Fig.

U.S.Cl 75/159, 75/153, 75/161, 136/241 Int. Cl C22c 9/06, l-lOlv 1/14 Field of Search 75/ 1 53, 159; 136/241 References Cited UNITED STATES PATENTS 7/1920 Fahrenwald 75/159 3,017,269 1/1962 Finch et a1. 75/159 FOREIGN PATENTS 625,523 8/1961 Canada 75/159 Primary Examiner-Charles N. Lovell Attorneys-Norman J. OM alley and Theodore C. Jay, .lr.

AEME IN MICROVOLTS PATENTEU 0R2] ml TEMPERATURE IN F INVENTOR. TEH PO WA N6 BY' m THERMOCOUPLE EXTENSION WIRE BACKGROUND OF THE INVENTION Thermocouples are commonly employed in high temperature measurement and control. One such thermocouple employs an alloy containing 95 percent nickel, balance small amounts of manganese, aluminum, silicon and cobalt as the negative thermoelement and an alloy of percent chromium, balance essentially nickel as the positive thermo-element. This thermocouple is known as a Type K thermocouple.

Thermocouples are usually spaced from the measurement or control instrumentation by distances which can be 6 feet or more, and thus extension wires are used to connect the thermocouples to the instrumentation. Because of the high cost of nickel and nickel based alloys, extension wires for Type K thermocouples can be composed of less expensive metals or alloys.

It is well known to use as extension wires, for the Type K thermocouple, an iron wire as the positive thermocouple extension wire and a copper-nickel alloy wire as the negative thermocouple extension wire. This last named alloy contains about 16 percent nickel, balance copper.

In the normal installation, the thennocouples are connected to a header junction, and the extension wires are connected between the junction and the instrumentation. In most industrial applications, the temperature of the header junction will not exceed 400 F. while the temperature at the instrumentation will not fall below 32 F.

Ideally, to avoid inaccuracy, the extension wires and the thermocouple should have matched thermoelectric characteristics at which the differential electromotivc force (EMF) developed between the two extension wires should be equal, both in polarity and magnitude, to the differential EMF developed between the two thermocouple wires at any temperature within the range 32 F. to 400 F.

l have found that the use of copper-nickel alloy wire as a negative thermocouple extension wire together with an iron wire as the positive thermocouple extension wire does not provide an ideal match for Type K thermocouples.

In my invention, I employ a new alloy composition for the negative extension wire which, when used together with an iron wire as the positive extension, produces a much better match for Type K thermocouples.

SUMMARY OF THE INVENTION In accordance with my invention, a negative thermocouple extension wire comprises, as expressed in percent by weight: 16-20 percent nickel; 1.30-2.00 percent manganese; 0.70-l.70 percent cobalt; balance copper. Small amounts of deoxidizers selected from the class consisting of boron, magnesium and silicon can be added to this composition.

The difi'erential EMF between an iron wire used as the positive thennocouple extension wire and my negative thermocouple extension wire, as measured over the temperature range 32 F. to 400 F. is substantially matched with the EMFs generated by Type K thermocouples.

BRIEF DESCRIPTION OF THE DRAWING The accompanying Figure is a graph showing the match between the differential EMFs of my invention and a Type K couple as a function of temperature in comparison with the existing copper-nickel alloy.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS My negative extension wire comprises 16-20 percent nickel; 1.30-2.00 percent manganese; 0.70-1.70 percent cobalt; balance copper. A first illustrative composition is 18 percent nickel; 1.35 percent manganese; 0.90 percent cobalt; balance copper, which can also contain, as deoxidizers, 0.04 percent silicon, 0.03 percent magnesium and 0.01 percent boron.

1n the Figure, curve 10 illustrates the temperature-EMF tially of as expressed in percent by weight:

Nickel l6%-20% Manganese l.30%2.00%; Cobalt 0109541095; and Balance Copper 

